Asteroids: What Are They and Where Do They Come From?

What Are They?

Asteroids are usually irregular shapes. This is Asteroid 243 Ida.
NASA / JPL

Asteroids are rocky objects primarily found in the asteroid belt, a region of the solar system that lies more than 2 ½ times as far from the Sun as Earth does, between the orbits of Mars and Jupiter. These objects are sometimes called minor planets or planetoids. They are likely the leftovers from the early formation of the solar system and their composition may shed light on what the early solar system was like. They probably formed from the protoplanetary disk that surrounded the Sun but never had enough mass to form into the roughly spherical shape required to be considered a planet. Despite thousands having been discovered, their total collective mass is still far less than the mass of the Earth.

Asteroids are meteoroids' big cousins: they range in size from 1 meter to larger than 100,000 meters, whereas meteoroids are generally smaller than 1 meter in diameter, but they are made from the same rocky or metallic materials.

A subset of these objects, the Trojan asteroids co-orbit the Sun with Jupiter and are in gravitationally stable points between the two, called Lagrange points. It has been estimated that there are as many Trojans as there are asteroids in the asteroid belt.

Asteroids' Encounters with Earth

Asteroids that fall into the category of Near Earth Objects have orbits that bring them in close proximity with our planet, and a number of notable impact sites have been attributed to asteroids. Most famously, the Chicxulub crater under the Yucatán Peninsula in Mexico is the result of an asteroid impact that might have wiped out the dinosaurs. Another asteroid known as Tunguska didn't impact Earth, but exploded a few miles above the Podkamennaya Tunguska River on June 30,1908. A similar explosion made headline news in 2013, when the near-Earth asteroid dubbed Chelyabinsk exploded in air blast that left 1,500 Russians seeking medical attention.

One thought on “Asteroids: What Are They and Where Do They Come From?

  1. equusmtn

    As an avid reader and watcher of science articles and television shows, I’m continually frustrated by the lack of a satisfying explanation for the formation of asteroids. Plenty of verbiage is devoted to explaining why the asteroid belt is located where it is, but almost none to how these small, solid objects formed in the first place. And the few sketchy words I’ve seen on the subject don’t have the “ring of truth” by a long shot.

    The explanation that asteroids simply coalesced from the gas/dust cloud from which the solar system originated is clearly wrong, or at least it leaves out several key steps. That theory — if it really is a theory anyone adheres to — fails to explain a) the differing compositions of asteroids (some rocky, some metallic, etc.), and b) how a small object with minimal gravity can condense from gas and dust into a solid object.

    If asteroids simply condensed directly from the primal gas/dust cloud, their composition would be much more homogenous, both within themselves and with each other. That fact seems too obvious to require any further explanation.

    Regarding their solidity, imagine the amount of compressive force it would take to smash gas/dust cloud particles “as fine as cigarette smoke” into a solid object. The gravity of even a relatively large asteroid is far too weak to generate that kind of compression. As I understand it, an object has to be many miles in diameter before its gravity is sufficient to establish solidity. Yet the vast majority of “space rocks”, even in the asteroid belt, are nowhere near this size.

    So what’s the answer? How did the space rocks we see form? Clearly solidity can only be achieved in large objects. And the only way to produce differentiation between rocks and metals is in the hot interior of a planet or protoplanet, where these substances melt, the heavier metals sink to the core, and the lighter rock-forming materials rise to the outer layers. My conclusion is that asteroids of any significant size — say, larger than grains of sand — are the remnants of planets/protoplanets that formed in the early solar system and which subsequently collided with each other and blasted chunks of their materials into space. Many of these chunks were maneuvered by gravitational perturbations into what is now the asteroid belt, while others were scattered around the solar system — some of which rain down on the earth and other planets every day.

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